Ink jet heads for recording information by utilizing a piezoelectric effect of piezoelectric elements have been known in the prior art, as disclosed in, for example, Japanese Laid-Open Patent Publication No. 5-18735. An ink jet head of this type is provided with actuators having piezoelectric elements, and is configured to discharge ink through nozzles by the action of the actuators.
Typically, a plurality of pressure chambers to which ink is supplied and a common ink chamber communicated to the pressure chambers are formed separately from each other in a head body. A plurality of nozzles respectively corresponding to the pressure chambers are formed on the reverse side surface of the head body. On the other hand, a vibration plate, a common electrode, a piezoelectric element and a separate electrode are deposited in this order on the front side surface of the head body, and the vibration plate, the common electrode, the piezoelectric element and the separate electrode together form an actuator for discharging ink through a nozzle by applying a pressure on the pressure chamber.
In order to drive the actuator, there is needed a driver IC, separately from the head body, for outputting a driving signal to the actuator. When the driver IC is provided on the printer body, it is necessary to extend the same number of driving signal lines as the number of nozzles from the printer body to the head body by using an FPC, or the like. Thus, there was a problem that the total length of the driving signal lines increases.
In view of this, as a technique for shortening the driving signal lines, there has been proposed a technique of providing the driver IC near the side surface of the head body (the surface perpendicular to the surface along which nozzles are arranged), and providing the same number of driving signal lines as the number of nozzles from the driver IC near the head body to the head body via an FPC, or the like. Moreover, in the ink jet head disclosed in Japanese Laid-Open Patent Publication No. 5-18735, supra, a driver IC 121 is mounted on a vibration plate 103 of a head body 100 as illustrated in FIG. 19, so that the only signal lines between the printer body and the head body are the signal lines for IC driving. Specifically, the driver IC 121 is mounted in parallel beside piezoelectric elements 102 and a common electrode 104. Note that in FIG. 19, 122 is a line pattern for connecting the driver IC 121 and separate electrodes to each other.
However, with the way of mounting disclosed in the above-identified publication, the driver IC 121 is simply directly mounted on the vibration plate 103 with no special modification. Therefore, it was necessary to arrange the driver IC 121 in parallel to and remotely from the piezoelectric element 102 so as to avoid the area of the vibration plate 103 where it actually vibrates (the area where the actuators 102 are provided). Stated conversely, it was necessary to ensure an additional space on the surface of the head body for mounting the driver IC 121. Moreover, since the driver IC 121 is provided remotely from the actuators 102 as described above, it was necessary to extend the lines 122 from the actuators 102 to the driver IC 121, thereby inevitably increasing the length of the lines 122. Therefore, the surface area of the head body 100 increased, and it was unavoidable for the ink jet head as a whole to be large in size. Note that such a problem similarly occurs in other arrangements where the driver IC is provided near a side surface of the head body.
Moreover, in the conventional head, driver IC 121 was made of a semiconductor material such as silicon, whereas the head body was made of a resin material, or the like. In such a case, the coefficient of linear expansion of the material of the driver IC and that of the material of the head body are substantially different from each other. For example, while the coefficient of linear expansion of silicon is 2.5×10−6[1/° C.], the coefficient of linear expansion of a resin material is larger than this by one order of magnitude or more. Therefore, in a case where the driver IC is mounted on the head body by flip chip bonding, wherein solder bumps, or the like, between terminals are melted by heating, contact failure between terminals was likely to occur due to the difference therebetween in the degree of thermal expansion. Moreover, even if a desirable connection was obtained when heated, thermal contraction occurred along with the subsequent decrease in temperature, resulting in peeling off of the terminals in some cases.
Particularly, the density of the head has recently been increased, whereby the interval between actuator terminals is becoming shorter and shorter. Thus, even a slight difference in the degree of thermal expansion and thermal contraction between the driver IC and the head body may lead to contact failure between terminals, thereby extremely reducing the yield of the product.
Moreover, the following problem exists which is characteristic of piezo type ink jet heads. That is, a piezo type ink jet head discharges ink by flexural deformation of actuators. Therefore, as the rigidity of the actuators changes, the ink discharging performance (e.g., the ink discharge velocity, the discharge amount, the driving frequency, etc.) changes. When the degree of thermal deformation of the driver IC differs from that of the head body, the head body (particularly, the actuators) undergoes a residual stress, i.e., a tensile shear force or a compression shear force, from the driver IC, whereby the rigidity of the actuators changes. Specifically, when an actuator undergoes a tensile shear force, the rigidity thereof increases and it becomes less flexible, whereas when it undergoes a compression shear force, the rigidity thereof decreases and it becomes more flexible. Thus, there was a problem that when the coefficient of linear expansion of the driver IC is substantially different from that of the head body, the rigidity of the actuators changes, thereby making the ink discharging performance instable.
Moreover, a difference in coefficient of linear expansion between the driver IC and the head body might possibly cause warping of the head body. As a result, the striking positions of ink droplets discharged from nozzles at both ends of the head body might possibly be shifted from the intended positions.
The present invention has been made in view of the above, and has an object to facilitate downsizing of an ink jet head.
Another object is to prevent contact failure between terminals and deterioration of the discharging performance due to thermal expansion and thermal contraction, thereby improving the reliability and the yield of a head.